Save Money! Avoid Unintended Charges
Turn off In-App Purchases to Avoid Accidental Charges!
In-App Purchases allow you to purchase items from WITHIN an app. For example, if you play a game that allows you to build things with “game gold” and you don't have enough game gold, the game will often offer you the option to purchase game gold for real money. If you don't want to accidentally purchase something, or if you have children who might purchase something without your permission, it's a good idea Turn Off In-App Purchases:
Go to Settings – General - Restrictions – enable Restrictions (if it isn't already)
It will prompt for creation of a code (or to enter the code if it is already on). I use the same code as the one that unlocks my phone, but if you intend the restrictions to be for a child, best to use something your kid doesn't know. Restrictions are the Parental Controls. Near the bottom of the Apps, turn off “In-App- Purchases.” I leave everything else on because I want full access to my account, but if you have a child you can also control things like installing / deleting apps, content rating restrictions, etc. here.
Limit Mobile Data to Avoid Unexpected Data Usage
Mobile data is crucial for using your phone when not on wifi, but you can save money by limiting the number of apps that use the data to the bare minimum.
Go to Settings – Mobile Data – and make sure only the apps you need are turned on.
Each app will have the recent data usage next to it. I also have WiFi Assist turned off. Wifi Assist automatically switches to Data if your wifi connection is slow, but it's mostly unnecessary and consumes battery. See my article on "Maximising Your Battery Life" for more battery-saving tips. I keep on things like Skype, Viber, Safari, Mail, Google Maps – anything I need to use to communicate with the world. Everything else is turned off. If you are concerned about your data (ie if you're approaching your limit), you can reset the Mobile Data statistics so you can see your exact data usage from that moment.